RXTE Discoveries

RXTE Finds Cannibal Star That May Be A Missing Link - July 1998

A newly-discovered star that is emitting rapid pulses of X-
rays may be the long-sought missing link between old neutron stars
that emit powerful flashes of X-rays, and older, rapidly spinning
neutron stars that emit mainly radio waves.

This star, designated SAX J1808.4-3658, is located 12,000 light years away
towards the constellation Sagittarius. It was found
by two competing teams of scientists using the Rossi X-ray Timing Explorer.
The first team, led by Dr. Michiel van der Klis and Rudy Wijnands of the
University of Amsterdam, the Netherlands, discovered the pulsar and measured
the time between rapid pulses of X-rays from the star to derive its
rotation rate. The second team, led by Dr. Deepto Chakrabarty and
Dr. Edward Morgan of the Massachusetts Institute of Technology,
Cambridge, MA, discovered the two-hour orbital period of the
pulsar and measured the size of the orbit, inferring the presence
of a companion star. The results are being presented in the July
23 edition of the journal Nature.

SAX J1808.4-3658 is not only an X-ray pulsar, it is the fastest-spinning
pulsar of its type ever seen. It has actually accelerated its own rotation
at the expense of its nearby "companion" star by pulling gas from the
companion onto its surface in a process called accretion. Its rotation rate
is upwards of 400 times a second, giving it a spin period of 2.5 milliseconds;
this translates to spinning once every 2.5/1000 of a second.
Scientists call pulsars with such short spin periods "millisecond pulsars";
this particular pulsar is the first known millisecond pulsar to power itself
by accretion.

This new pulsar helps scientists resolve a mystery. Prior to the discovery,
two populations of neutron stars with relatively weak magnetic fields but
with otherwise different characteristics were known. There were old,
accreting neutron stars, which generate X-rays from
the material they are gobbling up from their companions, and a group of
radio wave emitting millisecond pulsars that are rotating very rapidly and
slowing down gradually. Scientists suspected there was a connection between
the two, and the discovery of this pulsar that is both emitting X-rays and
spinning rapidly provides the link.

Although the magnetic fields of these two neutron star
types are much stronger than the Earth's field, they are
relatively weak by pulsar standards. Scientists think the weak
magnetic field allows the accretion process to spin the star up to
a high rotation rate. After the accretion phase, X-ray emission
from the pulsar ceases because there is no longer any infalling
material to generate X-rays. The rotation speed begins to slow
down at this point, because the accreting material was responsible
for keeping the spin up as well. The pulsar's magnetic field
rotates along with the star. The newly spun-up millisecond pulsar
starts to emit radio waves as subatomic particles from its surface
are accelerated into space by the pulsar's rotating magnetic
field.

Says Dr. van der Klis, "Astrophysicists have theorized for a long time that the
only reason millisecond pulsars exist at all is that they get spun
up by taking material from a companion star, but this is the first
time one has been caught in the act. This has sometimes been
called the Holy Grail of X-ray astronomy, and Rudy [Wijnands] has at last
found it!"

SAX J1808.4-3658, the "'stellar cannibal' is a leisurely diner," added
Chakrabarty. "We estimate that it has been pulling material from
its companion star for the last 100 million to one billion years.
Over that time, the companion star may have lost up to half its
mass. Currently, the companion is about 15 percent of the mass of
the Sun." However, not all the companion's mass loss is due to
accretion.

"Millisecond pulsars may throw away material they can't
capture by 'vaporizing' their companion stars with X-rays and
particle beams. As accreting gas falls on to the surface of the
pulsar, it heats up and emits X-rays. The X-rays blow material
from the companion star. After the accretion phase ends, the
pulsar may emit a high velocity beam of subatomic particles that
continues to blow material off the companion. Over a billion
years, this bombardment may cause the companion to vanish
altogether," said Dr. Tod Strohmayer, a member of the RXTE team
located at NASA's Goddard Space Flight Center, Greenbelt, MD.

"In the case of the newly discovered pulsar, we found that
its X-ray intensity is slightly fainter when it is on the far side
of its orbit (with its companion between us and the pulsar). This
is probably caused by an intervening 'fog' of material blown off
the companion's surface -- direct evidence for 'vaporization' by
the pulsar," said Chakrabarty.

Strohmayer added, "This X-ray and particle beam ablation may explain why
millisecond pulsars are often found alone, despite the fact that
they required a companion star to speed up. By 'vaporizing' the
companion, they hide the evidence - it's a stellar version of the
perfect crime."